U.S. Pat. No. 3,234,734 discloses a Rankine cycle power plant that operates on various organic fluids (e.g., biphenyl). The thermodynamic properties of many organic fluids are such that the saturated vapor line on the temperature-entropy (T-S) diagram for the fluids has a positive slope. As a result, working fluid that exits the vaporizer of a power plant in a saturated vapor state and is applied to the inlet of a turbine, expands and exits the turbine in a superheated state. Upon its expansion to the pressure of the condenser, the working fluid will contain a substantial amount of superheat that the condenser must remove before the working fluid can be condensed and returned to the vaporizer. The superheat rejected by the condenser into the ambient environment is wasted because the rejected heat, which was added to the working fluid in the vaporizer and preheater of the power plant, can not be converted to work.
The patent seeks to improve the thermodynamic efficiency of a power plant of the type described by decreasing the amount of superheat in the vaporized working fluid supplied to the condenser. It achieves this end by decreasing the superheat between the stages of turbine. Specifically, the patent discloses a multi-stage turbine in which vaporized working fluid exhausted from a preceding stage is withdrawn from the turbine, and mixed with liquid working fluid derived from a preheater interposed between the condenser of the power plant and the vaporizer. As a result, the superheat in the working fluid applied to the succeeding stage is reduced permitting this heat to be utilized for conversion to work instead of being rejected in the condenser. The result is an increase in the power produced by the power plant, and in its thermodynamic efficiency.
While the design approach in U.S. Pat. No. 3,234,734 is one that significantly improves the performance of a Rankine cycle power plant that operates on an organic fluid using a multistage turbine configuration, it does not address the economic problem of constructing a practical system to generate electricity in the range below, for example, about 10 MW with a low temperature heat source such as geothermal hot water with a temperature lower than about 400.degree. F. Thus, the patent shows a single multi-stage turbine, or a binary cycle using two turbines, which must be connected to a generator by way of a gear reducer because of the large differences in rotational speeds of the turbine and the generator. This configuration complicates construction, maintenance, and lubrication by reason of the multiple bearings and the gear reducer required.
It is therefore an object of the present invention to provide a new and improved expansion engine such as in a Rankine cycle power plant that utilizes an organic working fluid and which usually operates from a relatively low grade heat source of medium or low temperature fluid, which is less complicated and more reliable.